At the present time, wavelength division multiplexing (WDM) optical networks (ON) and various optical interconnecting devices (OID) represent one of the fastest growing fields of technology. Among other components, a typical WDM ON incorporates a wavelength demultiplexer, which discriminates and spatially separates individual optical channels, an array of photodetectors for converting optical signals of the individual optical channel into appropriate electrical signals, and amplifiers for amplifying the detected electrical signals for further data processing or data transmission.
In accordance with the existing practice, the aforementioned three components of an WDM ON are independently manufactured and assembled into a photoreceiving unit. Such systems are described, e.g., in “Understanding Optical Communications” by H. J. R. Dutton, 1998. A disadvantage of a photoreceiving unit assembled from three prefabricated devices is that it is large in size, expensive to manufacture, requires assembling operation, involves optical losses, makes it difficult to provide optical alignment, and is limited with regard to the number of optical channels. This is because an increase in the number of optical channels leads to rapid increase in cost and manufacturing complexity.
Attempts have been made to integrate some of aforementioned components into a monolithic optoelectronic integrated circuit (OEIC). Such a circuit is described, e.g., in: “Electronics Letters”, Vol. 35, No. 15, pp. 1248–1249. The device consists of a common substrate, which supports two individually manufactured components, such as a grating-type demultiplexer and an InGaAs photodetector array having individual photodetectors aligned with respective optical outputs of the demultiplexer. Since the aforementioned two components, i.e. the grating-type demultiplexer and the InGaAs photodetector array, are formed by growing and patterning on a common substrate, they partially solve the problems associated with optical losses and optical misalignment. Another advantage is that integration makes a tremendous step towards miniaturization.
However, the device described in the above reference still has a number of disadvantages such as absence of an amplifier in the form of an integrated component, channel cross-talk, and difficulties in manufacture associated with the formation of grating structures inside a grown semiconductor layer. It is also difficult to form a photodetector array accurately aligned with optical outputs.
The above problems can be partially solved by means of a device described in U.S. Pat. No. 5,689,122 issued in 1997 to S. Chandrasekhar. This device relates to a monolithic integrated demultiplexing photoreceiver that is formed on a semi-insulating InP substrate. A WGR unit of the device is formed on a common substrate and includes a first plurality of InP/InGaAs semiconductor layers. At least one p-i-n photodiode is separately formed on the same substrate and includes a second plurality of InP/InGaAs semiconductor layers. Additionally, at least one single heterostructure bipolar transistor is also separately formed on the same substrate and includes a third plurality of InP/InGaAs semiconductor layers. Two layers from each of the first, second and third plurality of layers are identical to one another. All three components are grown sequentially on the substrate, each in its individual process. In the photodetector, an electron-hole separating n-p plane is located parallel to the light propagation direction and is spaced from an optical signal transmission layer i.e. light waveguiding layer of WGR.
Thus, although all three aforementioned components, i.e., a WGR demultiplexer, a photodetector, and amplifier, are integrated on a common substrate, each of them has an individual layered structure different from the layered structure of other components. The manufacturing process is complicated, difficult to control, may have low repeatability, and is expensive as it requires three separate sequential manufacturing stages, one for each component. What is most important from the performance point of view is that the aforementioned position of the separating n-p plane parallel to the light propagation direction results in low optical coupling efficiency due to the fact, that, in order to reach the aforementioned plane or it vicinity, the light should change its direction by 90 degrees. This may result in significant light power losses at the photodetecting stage.